1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus, and more particularly, to a charger for charging the surface of a photoconductive drum by a contact charging method.
2. Description of the Related Art
An electrophotographic developing method is widely used in an image forming apparatus such as a copier, a laser beam printer, an LPH (LED print head) printer, and a plain paper facsimile machine. The electrophotographic developing method has the successive processes of: charging.fwdarw.exposing.fwdarw.developing.fwdarw.transfer.fwdarw.fixing.
In an electrophotographic image forming apparatus with a contact charger, a developing device includes a developing roller, a supply roller, a developing agent regulating blade and an agitator. A photoconductive drum, a charger roller, the developing roller, the supply roller, and a transfer roller rotate by means of a driving mechanism including an engine driving motor. A sheet is fed from a sheet cassette and conveyed along a sheet convey path and finally ejected to the exterior of the image forming apparatus.
Typically, the photoconductive drum is charged to a negative charge voltage by the charge roller and forms uniform charge on its surface. As the photoconductive drum rotates, its surfaces subject to an exposing process to form an electrostatic latent image. A non-image area of the surface of the photoconductive drum which has not been exposed to light maintains its original potential, while the potential of the exposed image area is decreased. The photoconductive drum forming the electrostatic latent image on its surface reaches a developing area as it rotates and the electrostatic latent image is developed into a visual image by a developing agent on the developing roller.
The developing roller has a negative developing potential and a negative bias voltage is supplied to the supply roller and the developing agent is charged to a negative potential by the difference in potential between the bias voltage of the developing roller and the bias voltage of the supply roller so as to adhere to the developing roller. The developing agent adhering to the developing roller is moved to an exposing area on the photoconductive drum due to the difference between the exposing potential of the photoconductive drum and the developing potential of the developing roller so that the developing agent adheres to the exposed area on the photoconductive drum.
The characteristic of the charge potential of the photoconductive drum charged by the charge roller varies according to the charge voltage of the charge roller and the environmental conditions. When the charge voltage applied to the charge roller is a DC voltage, the charge potential of the photoconductive drum is decreased as temperature and humidity are increased and is increased as temperature and humidity are decreased.
The developing agent on the developing roller is moved to the surface of the photoconductive drum by the difference in potential between the photoconductive drum and the developing roller. The amount of developing agent moved is proportional to the difference between the developing voltage and the charge potential of the photoconductive drum. Under the same charge voltage of the charge roller, the actual charge potential of the photoconductive drum differs according to environmental conditions. Therefore, the amount of developing agent on the photoconductive drum differs and thus image density varies, resulting in a degradation in image quality.
In order to prevent this problem, a charge voltage to which an AC component is superimposed has been applied to the charge roller so as to result in a charge potential with a relatively stable characteristic.
The characteristic of the charge potential of the photoconductive drum may differ when the charge roller is polluted by the developing agent or paper dust and if the charge roller is polluted, the charge potential of the photoconductive drum is lowered resulting in a unstable characteristic of charge potential with respect to the charge voltage of only a DC component. If the charge voltage includes AC component, the characteristic of the charge potential of the photoconductive drum becomes relatively stable. However, since voltage applied to the photoconductive drum and the charge roller varies according to an AC characteristic of the charge voltage, the electrostatic force therebetween differs. Then, the charge roller having an elastic characteristic due to the fact that it is normally formed on a rubber material, repeats contraction and expansion according to a period of an AC voltage frequency, thereby generating minute vibrations. The vibrations cause noise and a degradation in image quality. To prevent the vibration noise and to improve image quality, there is additionally needed vibration preventing device.
The following patents each discloses features in common with the present invention: U.S. Pat. No. 4,796,064 to Torrey, entitled Cycle-Up Control Scheme, U.S. Pat. No. 4,939,542 to Kurando et al., entitled Image Forming Apparatus With Potential Control, U.S. Pat. No. 5,412,455 to Ono et al., entitled Charging Device, Image Forming Apparatus And Detachably Mountable Process Cartridge Having A Constant Voltage Power Source Feature, U.S. Pat. No. 5,499,080 to Furuya et al. entitled Image Forming Apparatus Having A Voltage Controlled Contact Charger, U.S. Pat. No. 5,557,373 to Miyashita et al., entitled Cleaning System For Charging Drum Of An Image Forming Apparatus, U.S. Pat. No. 5,619,308 to Kinoshita et al., entitled Electrophotographic Image Forming Apparatus Adjusting Image Forming Means Based On Surface Voltage Of Photoconductor, U.S. Pat. No. 5,666,608 to Christensen, entitled Charging Member And Image Forming Member Space Apparatus, U.S. Pat. No. 5,659,839 to Mizude et al., entitled Voltage Control Apparatus For Controlling A Charger In An Image Forming Apparatus, U.S. Pat. No. 5,671,468 to Yamamoto et al., entitled Charging Member And Image Forming Apparatus Having Contact Charging Member, U.S. Pat. No. 5,697,013 to Sasaki, entitled Image Forming Apparatus For Forming Images In Accordance With An Electrophotographic Process, U.S. Pat. No. 5,715,499 to Yamazaki et al., entitled Contact Charger Having An Oscillating Voltage For Charging A Photosensitive Member, U.S. Pat. No. 5,749,022 to Kikui et al., entitled Charging Apparatus And Method For Use In Image Forming Device, and U.S. Pat. No. 5,768,653 to Fare, entitled Electrophotographic Printing Device With A Charging Roller.
Torrey '064 discloses a control device including a charging member, a sensor to measure the surface potential on an image bearing member--logic circuitry--means to determine relationship between charging current of the charging member and the measured surface potential of the image bearing member. Kuroando '542 discloses image forming apparatus with a potential sensor for measuring a potential at the surface of a photoreceptor drum--controlling an output from the charger. Ono '455 discloses a charging member--an image forming apparatus--a charging member for supplying the oscillating voltage to the charging member--a constant voltage element. Furuya '080 discloses an image forming apparatus having a voltage controlled contact charger. Miyashita '373 discloses an image forming apparatus including a charging member--a cleaning member--a controller is implemented by a microcomputer and controls various operations of the image forming apparatus. Kinoshita '308 discloses an electrophotographic image forming apparatus--a photoconductive drum--a voltage sensor for detecting a surface voltage--adjusted by an adjustment controller based on a surface voltage. Christensen '608 discloses an electrophotographic printing system--a spacer apparatus provides separation between a photoconductor drum and a charge roller. Mizude '839 discloses a voltage control apparatus for controlling a charger in an image forming apparatus--a controller corrects an output voltage value of the voltage value of the voltage generator to a voltage value necessary for the drum surface to be charged. Yamamoto '468 discloses a charging member and an image forming apparatus having a contact charging member. Sasaki '013 discloses image forming apparatus for forming images in accordance with an electrophotographic process. Yamazaki '499 discloses a contact charger having an oscillating voltage for charging a photosensitive member. Kikui '022 discloses a charging apparatus and a method for use in an image forming device--a controller controls an amount of the applied voltage. Fare '653 discloses an electrophotographic printing device with a charging roller for a photoconductive drum--a residual charged transferred by the charging roller to the drum can be controlled.
As noted above, earlier chargers lead to poor uniformity of image density according to a change in environmental conditions or the pollution of the charge roller. When the charge voltage including an AC component is applied to the charge roller, a vibration preventing device is required.